Device and process for determining the stiffness of sheet-like articles such as bank notes

ABSTRACT

The apparatus for determining the stiffness of sheet material is provided with mechanical means which periodically touch the sheet material, causing it to vibrate. The sounds produced by the vibration are detected by a detector. From the detected sounds an evaluating device determines the stiffness of the sheet material. The sheet material is held or guided in a transport device such that it can yield to the mechanical action within certain limits in the area of contact without being irreversibly deformed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus and method for determining thestiffness of sheet material such as bank notes.

2. Discussion of Related Art

Such an apparatus is known from EP-A 0 073 133. In this apparatus thesheet material is guided between two flat belts in the middle anddeflected via a double-conical roller by an angle of 180°.Simultaneously the sheet material is deformed in the longitudinal andtransverse directions. The sounds produced during this process aredetected by a microphone. From the detected sounds an evaluating devicedetermines the stiffness of the sheet material.

A disadvantage of the apparatus is that the produced sounds decreasegreatly when the stiffness of a sheet is measured several times. Thestiffness of the sheet material lessens with each measuring process dueto the deformation involved in the measuring process.

Starting out from this, the invention is based on the problem ofproposing an apparatus and method for determining the stiffness of sheetmaterial where the stiffness of the sheet material remains essentiallyunchanged.

SUMMARY OF THE INVENTION

The basic idea of the invention is essentially to produce sounds bymechanical means which periodically touch the sheet material, causing itto vibrate. The sounds produced by the vibrations are then detected by adetector. From the detected sounds an evaluating device determines thestiffness of the sheet material. Since the volume of the produced soundsis approximately proportional to the stiffness of the sheet material,the volume of the produced sounds is a direct measure of stiffness ofthe sheet material.

The sheet material is held or guided in a transport device such that itcan yield to the mechanical action within certain limits in the area ofcontact without being irreversibly deformed.

An advantage of the invention is that the stiffness of the sheetmaterial remains virtually unchanged. Even when the stiffness of a sheetis determined several times the intensity of the produced sounds remainsessentially the same. The reproducibility of the intensity of theproduced sounds is on the average at least 95%.

In a preferred embodiment of the invention the mechanical means have arotating roll with a certain number of corners. The corners are disposedin rotational symmetry on the roll. The sheet material is transportedthrough the apparatus by a transport device, and the driving axle of therotating roll is disposed perpendicular to the transport direction ofthe sheet material. The direction of rotation of the rotating roll is inthe direction of transport. The rotary frequency of the rotating roll isselected such that the circumferential speed dependent on the rotaryfrequency and the radius of the roll is greater than or equal to thetransport speed of the sheet material.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, various embodiments of the invention will be describedwith reference to the figures, in which:

FIG. 1 shows a schematic diagram of a preferred embodiment,

FIG. 2 shows a schematic diagram of the preferred embodiment withtransport belts,

FIG. 3 shows a schematic diagram of the preferred embodiment withtransport belts from below,

FIG. 4 shows a schematic diagram of the preferred embodiment withtransport belts and a guiding plate from below,

FIG. 5 shows a schematic diagram of a variation of the preferredembodiment,

FIG. 6 shows a schematic diagram of a second embodiment of theinvention,

FIG. 7 shows a schematic diagram of a third embodiment of the invention,

FIG. 8 shows a schematic diagram of a fourth embodiment of theinvention,

FIG. 9 shows a schematic diagram of a development of the fourthembodiment of the invention,

FIG. 10 shows a schematic diagram of a fifth embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 shows a schematic diagram of a preferred embodiment of theinvention is a side view. Sheet material 10 is transported through theapparatus in direction of transport T for example by means of transportbelts, which are initially not shown here for reasons of clarity. Themechanical means shown here for producing sounds are rotating roll 30with six rotationally symmetric corners. Driving axle 31 of rotatingroll 30 is aligned perpendicular to transport direction T of the sheetmaterial. A detector 20 detects the sounds produced.

The direction of rotation of roll 30 is in transport direction T of thesheet material. The rotary frequency of roll 30 is selected such thatthe circumferential speed of roll 30 dependent on the rotary frequencyand the radius of roll 30 is greater than or equal to the transportspeed of the sheet material. This selection of rotary frequency anddirection of rotation prevents the sheet material transported throughthe apparatus from hitting a corner of the roll and being slowed down.Such a slowdown would increase the danger of jamming in the transportsystem. In principle the operation of the apparatus can also be ensuredupon a change of direction of rotation or rotary frequency of roll 30.

Due to the rotation of roll 30 the corners periodically touch sheetmaterial 10, causing it to vibrate. The sheet material is thereby raisedin the area of roll 30 by lifting height H. Lifting height H is ameasure of the volume of the produced sounds.

The radius of roll 30, number N of the corners and the rotary frequencyof roll 30 are the parameters by means of which the preferred embodimentof the invention can be optimally adapted to the particularapparatus-related boundary conditions, such as maximum spatial extent ofthe apparatus. The contact frequency of the roll on sheet material 10results from the product of rotary frequency and number of corners.Lifting height H depends on the selected roll radius and number ofcorners.

FIG. 2 and FIG. 3 show the preferred embodiment with a transport devicein a side view and from below, the transport device being integratedinto hexagonal roll 30. The transport device has three upper transportbelts 11 and two lower transport belts 12 between which sheet material10 is clamped for transport through the apparatus.

To avoid interactions between rotating roll 30 and upper transport belts11 or lower transport belts 12, free-running rollers 32, 33 across whichthe corresponding transport belts are guided are provide din the roll.These rollers can be decoupled from driving axle 31 for example bycorresponding ball bearings so that transport belts 11 and 12 can bemoved at constant transport speed independently of the circumferentialspeed of roll 30. The radius of free-running rollers 32 is preferablyselected equal to the radius of the incircle of roll 30. The radius offree-running rollers 33 is preferably selected smaller than the radiusof the incircle of hexagonal roll 30. If no sheet material 10 is presentthe upper edges of lower transport belts 12 and the lower edges of uppertransport belts 11 are flush with roll 30 in the position of roll 30according to the upper part of FIG. 1. The contact of roll 30 with sheetmaterial 10 takes place in the areas between transports belt 11 and 12.

FIG. 4 shows the preferred embodiment with upper transport belts 11 andguiding plate 13, regarded from below. Guiding plate 13 functionallyreplaces lower transport belts 12 according to FIG. 3. During transportthrough the apparatus, sheet material 10 is transported by uppertransport belts 11 across guiding plate 13. Here, too, transport belts11 are decoupled from roll 30 by free-running rollers 32. Guiding plate13 has gap 14 through which roll 30 comes in contact with sheet material10.

FIG. 5 shows a variation of the preferred embodiment with four-corneredroll 34. In contrast to hexagonal roll 30 shown in FIG. 1, liftingheight H of four-cornered roll 34 is greater than lifting height H ofhexagonal roll 30. If the other parameters are constant, the soundsproduced by four-cornered roll 34 are thus louder than those fromhexagonal roll 30. In order to ensure a constant contact frequency inboth variants of the embodiment, the rotary frequency of four-corneredroll 34 must be increased accordingly over hexagonal roll 30. The rotaryfrequency can only be varied within certain limits due to the mechanicalproblems occurring at high rotary frequencies, however, so that it isnecessary to optimize the parameters in order to produce sufficientlyloud sounds at a given contact frequency.

FIG. 6 shows a second embodiment of the invention wherein brushes 41 aredisposed in rotational symmetry on rotating roll 40. Brushes 14 arepreferably executed to be movable on their own axes, so that brushes 41swing outward due to the centrifugal force occurring upon rotation ofroll 40. The sounds necessary for determining the stiffness of the sheetmaterial are produced here by the periodic contact of brushes 41 againstsheet material 10. The parameters described for the preferred embodimentof the invention, and the shown variations, can be easily transferred tothis embodiment so that another description can be dispensed with.

FIG. 7 shows a third embodiment of the invention wherein the device forproducing sounds has as mechanical means electromagnet 50 with yoke 51,coil 52 and movable tongue 53. By applying an alternating voltage tocoil 50 one causes movable tongue 53 to vibrate. The sounds are producedby periodic touching of sheet material 10 by tongue 53. The vibratingfrequency of movable tongue 53 corresponds to the frequency of theapplied alternating voltage. Lifting height H can be varied by themaximum voltage difference of the alternating voltage.

FIG. 8 shows a fourth embodiment of the invention wherein the mechanicalmeans for producing sounds have piezoelectric element 60. By applying analternating voltage one can vary the volume expansion of piezoelectricelement 60 in the direction perpendicular to the plane of the sheetmaterial. The frequency of expansion of piezoelectric element 60corresponds to the frequency of the applied alternating voltage. Theextent of expansion of piezoelectric element 60 depends on the maximumvoltage difference of the alternating voltage, but is relatively low.This leads merely to small lifting height H of sheet material 10 andthus also to a low volume of produced sounds.

A method for increasing lifting height H of sheet material 10 is shownin FIG. 9. It consists in providing additional lever system 70 connectedwith piezoelectric element 60 and used for increasing the expansion ofpiezoelectric element 60 so as to obtain desired lifting height H.

Alternatively one can use for example so-called bimorphic piezoelectricelements 60 to increase lifting height H of sheet material 10. In suchbimorphic piezoelectric elements 60 at least two piezoelectric elements60 are firmly interconnected and the expansion of piezoelectric elements60 is converted into a bending of piezoelectric elements 60. Bimorphicpiezoelectric elements 60 can produce lifting heights H of severalmillimeters.

FIG. 10 shows a fifth embodiment of the invention wherein the mechanicalmeans for producing the sounds have coil 80 with movable magnetic core81. When one applies an alternating voltage to the coil, movablemagnetic core 81 vibrates at the frequency of the alternating voltage.The sounds arise through periodic touching of sheet material 10 bymagnetic core 81. The excursion of movable core 81 and thus liftingheight H of sheet material 10 can be influenced by the maximum voltagedifference of the alternating voltage.

In addition to the above-described embodiments, it is of course possiblefor the expert with knowledge of the basic idea of the invention todevelop variations of the described embodiments or new embodiment basedon the idea of the invention. It is specifically possible to transferthe transport devices explained for the preferred embodiment to theother embodiments analogously.

All known methods of analog or digital signal processing can be used forevaluating the detected sounds.

We claim:
 1. In an apparatus for determining the stiffness of sheetmaterial such as bank notes, includinga device for producing soundsusing the sheet material, a detector for detecting the produced sounds,an evaluating device for determining the stiffness of the sheet materialfrom the detected sounds, the improvement comprising:said device forproducing sounds comprising a mechanical device arranged so that it (30,34, 40, 41, 50, 60, 70, 80, 81) periodically touches the sheet material(10) and periodically raises it by a lifting height (H) above anon-raised height and permits the sheet material to be periodicallylowered to its non-raised height to thereby cause it to vibrate.
 2. Theimprovement of claim 1 wherein the mechanical device comprises arotating roll (30, 34) with a selected number (N) of rotationallysymmetric corners.
 3. The improvement of claim 1 wherein the mechanicaldevice comprises a rotating roll (40) with brushes (41) disposed inrotational symmetry thereon.
 4. The improvement of claim 2, wherein thesheet material is transported through the apparatus by means of atransport device (11, 12), and the driving axle (31) of the rotatingroll (30, 34, 40) is disposed perpendicular to the transport direction(T) of the sheet material.
 5. The improvement of claim 4, wherein thecircumferential speed of the roll (30, 34, 40) is in the direction oftransport (T) of sheet material and is greater than or equal to thetransport speed of the sheet material.
 6. The improvement of claim 4,wherein the transport device has upper and/or lower transport belts(11,12) decoupled from the driving axle (31) by free-running rollers(32,33).
 7. The improvement of claim 6, wherein the transport device hasa guiding plate (13) with a gap (14) through which the roll (30, 34, 40)comes in contact with the sheet material (10).
 8. The improvement ofclaim 1, wherein the mechanical device comprises an electromagnet (50)with a movable tongue (53).
 9. The improvement of claim 1, wherein themechanical device comprises a piezoelectric element (60).
 10. Theimprovement of claim 9, wherein the mechanical device comprises anadditional level system (70) connected with the piezoelectric element(60).
 11. The improvement of claim 9, wherein the mechanical devicecomprises at least one further piezoelectric element (60) and thepiezoelectric elements (60) are firmly interconnected so that theexpansion of the piezoelectric elements (60) is converted into a bendingof the piezoelectric elements.
 12. The improvement of claim 1, whereinthe mechanical device comprises a coil (80) with a magnetic core (81).13. In a method for determining the stiffness of sheet material such asbank notes, including producing sounds using the sheet material, anddetecting the produced sounds of the sheet material, whereby thestiffness of the sheet material is determined from the detected soundsof the sheet material, the improvement comprising:producing the soundsusing the sheet material (10) by periodic touching the sheet material(10) to periodically raise the sheet material a lifting height (H) abovea non-raised height and permitting the sheet material to periodicallydrop to its non-raised height to thereby cause the sheet to vibrate.